Given that the European Union is essentially a large bureaucracy, one would expect Mr. Selmayr’s back-room skill to be admired. But he has made numerous enemies on the way up, and this latest sleight of hand has produced outrage in the Brussels news corps, elicited unusual animosity from the bloc’s media officers and prompted calls from European lawmakers for an investigation.

The European Parliament is scheduled to debate the issue on Monday.

The strong reaction to the appointment has been led by Jean Quatremer, a Brussels correspondent for the French newspaper Libération since 1990 and the author of a blog called Coulisses de Bruxelles, roughly meaning Backstage Brussels.

Mr. Quatremer, 60, has no great liking for Mr. Selmayr, whom he has described at various points as a monster, a Rasputin, an authoritarian, a “man of the shadows more feared than loved,” who “does not hide his contempt for the mediocre” and who has often made his colleagues cry. He has said that Mr. Selmayr acts like “a de facto deputy president.”

But his efforts, backed up by much of the European news pack, to get responses from Mr. Juncker, Mr. Selmayr or others have been largely rebuffed with increasingly annoyed assurances that everything was in order.

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Jean-Claude Juncker, right, the European Commission president, is expected to step down next year.Credit
Dimitar Dilkoff/Agence France-Presse — Getty Images

Things got almost intense at the normally sedate and somnolent midday briefing last week, with the chief commission spokesman, Margaritis Schinas, insisting that “all the procedures, and I repeat all the legal procedures, under the staff regulations have been respected religiously.” Exasperated, he went on: “I repeat — respected religiously. It was all done by the book, by the rules.”

“These institutions don’t belong to you,” Mr. Quatremer responded. “They belong to the European citizens, and it is our perfect right to ask you questions, to repeat the questions as often as we want, without you giving us lessons in morality.”

The commission’s president normally decides who will be secretary general, but traditionally, those chosen have had high-ranking jobs in the European civil service. Mr. Selmayr came to Brussels in 2004 as a spokesman for Viviane Reding, also from Luxembourg, who was then the commissioner for society and the media. He has never been a deputy director general or director general in any of the commission’s departments.

What made the whole business seem slightly shady was that no one had been aware of the vacancy, or that the chief of staff, Alexander Italianer, 61, was contemplating retirement. Then it emerged that Mr. Selmayr had been appointed deputy secretary general briefly to qualify him to become secretary general.

And then it emerged that rather than an open competition, there had been only one other candidate for the vacancy of deputy secretary general — and that person withdrew before the selection process was completed. While the commission has refused to name the other candidate, Mr. Quatremer reported that it was Clara Martínez Alberola, Mr. Selmayr’s deputy in Mr. Juncker’s office, who has now succeeded him as Mr. Juncker’s chief of staff.

By the rules, there had to be a minimum of two candidates — including at least one man and one woman — for the deputy secretary general’s job. So the rules were followed, in form if not in spirit. Mr. Italianer has since been named a “special adviser” until his retirement. And Irene Souka, the director general in charge of human resources, who needed to approve all of this, and her husband, Dominique Ristori, who is the director general for energy, have both been allowed to extend their jobs past the normal retirement age.

“Selmayr had served Juncker well — or was it the other way around?” Mr. Quatremer wrote. “Juncker had handed the keys of the European house to his favorite Eurocrat,” Mr. Selmayr.

“Europe does not need these kind of issues now,” said Christian Lequesne, a politics professor at the Paris Institute of Political Studies, or Sciences Po, who specializes in European Union institutions and diplomacy. It is “news that is excellent for all euroskeptics,’’ he said.

“We are in a moment where the E.U. is contested in all member states, and public opinion learns that the E.U. commission is using such strange practices to appoint the secretary general,’’ Mr. Lequesne added.

Part of the angry reaction is also because of the preponderance of Germans in top jobs in the European bureaucracy. There are already Germans in the jobs of secretary general of the European Parliament (Klaus Welle) and secretary general of the European External Action Service, or foreign service (Helga Schmid). While no one doubts their competence or their European mind-set, the other 27 member states do want their share of the top jobs.

In late February, asked for an interview with Mr. Selmayr, the commission’s deputy spokeswoman, Mina Andreeva, said in an email that, given his “busy schedule,” nothing would be possible before Easter. Asked then for a specific appointment for an interview sometime after Easter, she said to “check back” closer to the time.